Nylon abrasive filaments were developed in the late 1950's as a man made alternative to natural abrasive filaments. At about that time an extrusion process was developed for dispersing abrasive particles uniformly in a nylon matrix in the form of a filament (U.S. Pat. Nos. 3,522,342 and 3,947,169). A review of nylon abrasive filaments is presented by Watts, J. H., "Abrasive Monofilaments-Critical Factors that Affect Brush Tool Performance", Society of Manufacturing Engineers Technical Paper, 1988, a written version of a presentation by the author at the WESTEC Conference, held Mar. 21-24, 1988. As explained by Watts, as filaments of this type wear, new abrasive particles are exposed. An abrasive filament brush tool made using a plurality of these filaments is thus regenerated during use. Some of the advantages of nylon abrasive filaments are their safety, cleanliness, cutting speed, low cost, superior radius and finish control, adaptability, and ease in design.
A key property of nylon and other thermoplastic materials is its "memory". In a brush filament this is referred to in the art as "bend recovery", or the tendency for a deflected filament to return to its original deployment. The bend recovery for nylon is generally over 90%, i.e., the filament returns to about 90% of its original deployment after being deflected.
Over time in operation, such as in a brush tool, most abrasive-filled polymeric filaments will take a set shape, and unless the filaments of the brush tool recover, the brush tool becomes soft and loses its effectiveness. Bend recovery is determined by filament diameter, relaxation time, strain, deflection time, and environmental conditions. Among synthetic filaments made to date, nylon offers the best bend recovery from strain held for an extended period of time.
While adequate for many purposes, the inventors herein have found that the various nylons have property limitations which make their use less than optimal in abrasive filaments. Nylon abrasive filaments have limited stiffness and may lose their stiffness as filament temperature approaches 70.degree. C., and thus may not be suitable for removing heavy scale or burrs when elevated filament temperatures are developed. Temperature resistance is critical in maintaining filament stiffness. Elevated temperatures generally affect all nylon polymers in a similar way: stiffness, as measured by the bending (tangent) modulus, decreases as temperature increases. Heat generation is normally not a problem in long filament deburring where brush tool speeds are low. However, in short trim power brushes, tool pressure on the part and/or high speed in a dry environment can generate high temperatures at the filament tips.
Another limitation of nylon abrasive filaments is that moisture from any source can have a noticeable affect on nylon filament brush tool performance. Moisture affects filament stiffness and thereby tool aggressiveness. Nylon 6,12 retains stiffness better than other nylon materials and is 2-3 times stiffer than other types of nylon in high humidity or when saturated with oils, solvents or when water is present.
In all abrasive filled polymeric filaments, as the degree of abrasive loading increases, the tensile strength and flex fatigue resistance tend to decrease, due to insufficient binding of abrasive and polymer. Bending modulus for a filament can be simply defined as the resistance to bending. This is an inherent characteristic of the polymer used for the abrasive filament. Bending modulus is generally independent of the filament diameter, and since the bending modulus of a family of abrasive filaments made from the same polymer will be the same, the main characteristics which affect filament stiffness are the diameter and length of the filament.
The abrasive cutting ability of abrasive-filled nylon filaments exhibits the distinct characteristic of cutting relatively well at the onset of the operation, followed by clear loss of abrasive action within about 1 hour. FIG. 7 shows the degradation in cutting ability of abrasive-filled nylon filaments, filled with a typical aluminum oxide abrasive, when the filaments are attached to a hub to form a brush and the hub rotated so that the filaments strike (and therefore abrade) a stationary workpiece. FIG. 7 represents the cut obtained on a flat carbon steel (1018) plate as a function of time at a constant load of 1.36 Kg. Equipment is typically designed to reverse the brush operation to restore the abrasive action to its original level of activity. An abrupt increase in cut can be achieved if the brush is "dressed", for example, by operating the brush against a wire screen. This is shown at 2 hours 15 minutes in FIG. 7. Another problem associated with abrasive-filled nylon filaments is their poor flex fatigue resistance. Over extended periods of operation the filaments tend to break near the point of attachment to the hub, an inconvenience to the user, resulting in decreased life and economic value of the brush.
The present invention addresses some of the problems mentioned above with abrasive-filled nylon and other filaments by presenting a composite abrasive filament comprising a preformed core coated with an abrasive-filled thermoplastic elastomer. This approach centers on the idea that a preformed core coated with an abrasive sheath has a higher initial bending modulus, a more constant bending modulus as a function of time, temperature, humidity and chemical environment, and higher tensile strength than an abrasive-filled thermoplastic filament.
Composite abrasive filaments having a preformed core are to be distinguished structurally from filaments comprising either an abrasive-filled core or sheath wherein the core and sheath are typically co-extruded and have similar mechanical properties, such as tensile strength. (These latter "in situ" core filaments are the subject of assignee's co-pending application, cross-referenced above.) Tensile strength may be significantly higher in composite abrasive filaments due to the tensile strength of the preformed core. Composite abrasive filaments may allow for up to twice the loading of abrasive grains into the thermoplastic elastomer coating without exhibiting significantly reduced flex fatigue resistance compared with abrasive-filled nylon filaments. Much higher levels of initial and continued abrasive action were observed than would have been expected from the increase in abrasive loading. This behavior relates to the compositional nature of the thermoplastic elastomers as well as to the method of preparation of the composite abrasive filaments.
Experimentation with and production of abrasive filaments has a long history. U.S. Pat. No. 2,328,998, Radford, discloses abrasive tools made from monofilaments containing abrasive particles either throughout the filament or in the sheath or core of a sheath-core structure. The filament may be made of cellulose ester, resins, or thermoplastic polymers (for example, nylon). The use of thermoplastic elastomers is not taught or suggested.
U.S. Pat. No. 2,643,945, Buckner, describes a device wherein a cotton cord is coated with abrasive grains using a furfuryl resin and then wound convolutely onto a core to produce a grinding or cut-off wheel.
U.S. Pat. No. 2,793,478, Rohowetz, describes abrasive filaments comprising a core consisting of a single strand or twisted, woven group of strands of metal, glass, or synthetic polymer, with a layer of flexible thermosetting resinous material permanently bonded to the core, and particles of abrasive material permanently embedded in the resinous material. A second embodiment describes a core, a layer of thermoplastic adhesive, a layer of thermosetting resin permanently bonded to the thermoplastic layer, with abrasive grains permanently embedded in the thermosetting layer. The use of thermoplastic elastomers is not taught or suggested.
U.S. Pat. No. 2,920,947, Burk et al., describes a core-sheath composition of a linear polyamide bristle having a surface coating of synthetic linear polyamide in which exposed solid abrasive particles are embedded, the particles being held in position by adhesion. A method for preparation is also presented which comprises coating a bristle with an aqueous dispersion of linear polyamide containing 5-50% abrasive, and drying the coating above 100.degree. C.
U.S. Pat. No. 3,146,560, Hurst, describes abrasive filaments comprising preformed synthetic filaments coated with a binder containing abrasive particles. The abrasive coated filaments are used to make abrasive articles. The synthetic filaments are typically a plurality of strands, each of which is formed from glass fibers that are twisted together. The binder is preferably a phenolic resin but can also include animal glue, compounded neoprene, and the like, or a synthetic resin such as resorcinol-formaldehyde resin or an aniline-formaldehyde, polyester, silane, epoxy or polyurethane resin. The use of thermoplastic elastomer binders is not taught or suggested.
U.S. Pat. No. 3,260,582, Zimmer et al. describes nonwoven polishing and abrading pads formed using long, continuous filaments of preformed and crimped synthetic cores coated with adhesives containing abrasive grains. Preferred preformed cores are polyamides, such as nylon, or polyester filaments. Other preformed cores disclosed are those capable of being thermoformed including the vinylidenes, olefins, fluorocarbons, acrylonitriles and acrylics. Adhesives may vary from the elastomeric to the hard, heat-advancing resinous type such as the polyurethane or phenol-aldehyde based adhesives. Again, the use of thermoplastic elastomers is not suggested.
U.S. Pat. No. 3,522,342, Nungesser et al., (mentioned above) describes apparatus and methods for making abrasive bristles having an abrasive filler, the apparatus utilizing two extruders. The method comprises melting a thermoplastic material in a first extruder and adding the abrasive filler to the molten thermoplastic through a second extruder, and extruding the mixture through a die which directs the output into a cooling water bath. Typical thermoplastic materials disclosed as useful include the nylons, polypropylene, polycarbonate, acetals, acrylics, polyethylene, polyurethane, polyvinylchloride, and combinations of nylon and a polyester, etc. The use of thermoplastic elastomers is not taught or suggested.
U.S. Pat. No. 3,547,608, Kitazawa, describes a method of manufacturing an impregnated fibrous grinding article, the method comprising feeding abrasive particles and a thermosetting resinous binder into the center of a rotary woven yarn while the yarn is rotary driven. After curing the binder, the composite is formed into a grinding article.
U.S. Pat. No. 3,669,850, Draca, describes an abrasive brushing element comprising a wire bristle having an outer layer of metal that binds very fine abrasive powders. An abrasive powder is electrostatically attracted to the tips of the wire bristles followed by electroplating the bristles with nickel.
U.S. Pat. No. 3,696,563, Rands, describes a brush comprising flexible filaments made from twisted metal or other appropriately flexible and heat resistant materials with a globule of abrasive filled material attached to the tip of each filament.
U.S. Pat. No. 3,854,898, Whitney, Jr., et al., describes automated methods for producing armored rods or aggressively coating a rod or wire substrate with a slurry of a flux paste adhesive and brazing metal powders, overcoating the latter with abrasive particles, followed by fusion of the brazing metal coating via heat.
U.S. Pat. No. 4,097,246, Olson, describes a method of making an abrasive wire for sawing stone, the abrasive wire comprising a support element such as a wire cable which is periodically coated with larger diameter elements which are coated with abrasives.
U.S. Pat. No. 4,172,440, Schneider et al., describes cutting filaments consisting of a linear monofilament of PET polyester into which from 0.3-10% by weight of an abrasive is incorporated. In the cutting process the abrasive particles are apparently not ripped off, as sometimes happens with steel wire cores bearing an abrasive on their surface, but the abrasive particles are said to be pressed into the monofilament.
U.S. Pat. No. 4,507,361, Twilley et al., describes low moisture absorption bristles of nylon and polyester. The bristles have a diameter of about 0.05-0.23 cm and are composed of about 10-30 wt. % polyamide based on total thermoplastic weight. The polyamide preferably has less than about 35% of its end groups being amine groups. The balance of the thermoplastic weight comprises polyethylene terephthalate having an intrinsic viscosity of at least 0.60. About 5 to about 50 wt. % of abrasive filler is included in each bristle.
U.S. Pat. No. 4,627,950, Matsui et al., describes a method of making a conjugate fiber comprising at least one layer composed of a polymer containing at least 20% by weight of abrasive particles and at least one coating layer substantially covering the abrasive layer. The coating layer is composed of a polymer containing substantially no abrasive particles. At least part of the coating layer apparently must be removed from the conjugate fiber (prior to its use as an abrasive filament) with a solvent to expose at least a part of the abrasive layer. Typical polymer coating layers include PET and nylon 6.
U.S. Pat. No. 4,585,464, Haylock et al., describes a low moisture absorption abrasive bristle of polybutylene terephthalate. The thermoplastic matrix comprising polybutylene terephthalate contains an abrasive filler and the bristles are made by the process of U.S. Pat. No. 3,522,342, mentioned above. The bristles are preferably stretched to a length about 2-4 times their extruded length for optimal tensile modulus and bend recovery.
U.S. Pat. No. 4,866,888, Murai et al., describes a wire encrusted with abrasive grains, produced by preparing a cylindrical metallic body having a metallic rod inserted into the central part of a metallic pipe, with a gap formed between the rod and the pipe, then filling the gap with a mixed powder comprising a metallic powder and abrasive grains. This structure is then hot and cold worked before removing the above-mentioned outermost metallic pipe.
U.S. Pat. No. 5,068,142, Nose et al., describes a fiber-reinforced polymeric resin composite material comprising a thermoplastic polymeric resin matrix which impregnates and covers a number of individual reinforcing fibers. Thermoplastics include nylon 6, nylon 66, polyolefins, polyesters, and others, while reinforcing fibers include carbon, glass, aramid, stainless steels, copper, and amorphous metal fibers. The composites do not contain abrasive material, nor is the use of thermoplastic elastomers taught or suggested.
French Patent Application No. 2,624,773, published Jun. 23, 1989, Ferrant et al., describes an abrasive wire consisting of a core made up of a man-made fiber such as an aramid fiber which acts as the strength member of the fiber. The core is then covered with an abrasive material that is held in place by a thermosetting resin binding agent that has been previously applied. The surface is then wrapped with a binding thread and a second coating of abrasive material is applied.
European Patent Application No. 0 282 243, published Sep. 14, 1988, Susa et al., describes abrasive filaments made of a composition which comprises 95-70 volume percent of a polyvinylidene fluoride resin, whose inherent viscosity ranges from 0.9 to 1.4, and 5-30 volume percent of abrasive grains. The abrasive filaments are produced by melt-spinning the composition and then stretching the resulting filaments at a draw ratio of 2.5 times--5.5 times within a temperature range of 100.degree.-200.degree. C.
It should be clear at this point that Applicant does not contend that he has been the first to incorporate abrasive grains into a plastic or resinous filament. It should also be clear that there is a distinction between filaments having a core-sheath arrangement and filaments having a preformed core coated with a plastic material filled with abrasive grains. The present invention is concerned with composite abrasive filaments comprising preformed cores at least partially coated with abrasive-filled thermoplastic elastomer compositions, which have the unexpected properties of allowing up to twice the loading of abrasive grains into the binding polymeric sheath while exhibiting many times the flex fatigue life compared to previously known filaments. Much higher levels of abrasive action were observed than would have been expected from the simple increase in abrasive loading.
Thermoplastic elastomers are defined and reviewed in Thermoplastic Elastomers, A Comprehensive Review, edited by N. R. Legge, G. Holden and H. E. Schroeder, Hanser Publishers, New York, 1987 (referred to herein as "Legge et al", portions of which are incorporated by reference hereinbelow). Thermoplastic elastomers (as defined by Legge et al. and used herein) are generally the reaction product of a low equivalent weight polyfunctional monomer and a high equivalent weight polyfunctional monomer, wherein the low equivalent weight polyfunctional monomer is capable on polymerization of forming hard a segment (and, in conjunction with other hard segments, crystalline hard regions or domains) and the high equivalent weight polyfunctional monomer is capable on polymerization of producing soft, flexible chains connecting the hard regions or domains. This type of material has not been suggested for use in abrasive filaments.
"Thermoplastic elastomers" differ from "thermoplastics" and "elastomers" (a generic term for substances emulating natural rubber in that they stretch under tension, have a high tensile strength, retract rapidly, and substantially recover their original dimensions) in that thermoplastic elastomers, upon heating above the melting temperature of the hard regions, form a homogeneous melt which can be processed by thermoplastic techniques (unlike elastomers), such as injection molding, extrusion, blow molding, and the like. Subsequent cooling leads again to segregation of hard and soft regions resulting in a material having elastomeric properties, however, which does not occur with thermoplastics.
Some commercially available thermoplastic elastomers include segmented polyester thermoplastic elastomers, segmented polyurethane thermoplastic elastomers, segmented polyurethane thermoplastic elastomers blended with other thermoplastic materials, segmented polyamide thermoplastic elastomers, and ionomeric thermoplastic elastomers.
"Segmented thermoplastic elastomer", as used herein, refers to the sub-class of thermoplastic elastomers which are based on polymers which are the reaction product of a high equivalent weight polyfunctional monomer and a low equivalent weight polyfunctional monomer.
"Ionomeric thermoplastic elastomers" refers to a sub-class of thermoplastic elastomers based on ionic polymers (ionomers). Ionomeric thermoplastic elastomers are composed of two or more flexible polymeric chains bound together at a plurality of positions by ionic associations or clusters. The ionomers are typically prepared by copolymerization of a functionalized monomer with an olefinic unsaturated monomer, or direct functionalization of a preformed polymer. Carboxyl-functionalized ionomers are obtained by direct copolymerization of acrylic or methacrylic acid with ethylene, styrene and similar comonomers by free-radical copolymerization. The resulting copolymer is generally available as the free acid, which can be neutralized to the degree desired with metal hydroxides, metal acetates, and similar salts. A review of ionomer history and patents concerning same is provided in Legge et al., pp. 231-243.
The benefits of thermoplastic elastomers, including ease of processability combined with hard rubber characteristics, have given some unexpected abrasive binding and cutting properties. Composite abrasive filaments of the present invention comprising preformed cores and abrasive-filled thermoplastic elastomer coatings produce much higher levels of initial cut, maintain their higher cutting ability once an equilibrium condition has been achieved, and are much more resistant to flex fatigue failure than abrasive-filled nylon filaments.